#### Answer

$\cos 9x$

#### Work Step by Step

$\cos \dfrac {A}{2}=\pm \sqrt {\dfrac {1+\cos A}{2}}\Rightarrow \pm \sqrt {\dfrac {1+\cos 18x}{2}}=\cos \dfrac {18x}{2}=\cos 9x$

Published by
Pearson

ISBN 10:
013421742X

ISBN 13:
978-0-13421-742-0

$\cos 9x$

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